Effects of thermal and physical treatments on donkey milk nutritional properties

Donkey milk (DM) may be considered a good and safe replacer to other kinds of milk for patients affected by cow's milk protein allergy (CMPA), which is an abnormal immunological reaction to cow milk proteins, resulting in immediate IgE-mediated reactions [1]. An important attribute of DM is its chemical resemblance to human milk especially regarding the lactose and the protein content, and also mineral composition [2]. The hypoallergenic properties of DM are due to the low content of αs1-and β-caseins and the presence of αs2- and κ-caseins trace levels [3]. Furthermore, DM is characterized by the presence of bioactive proteins (lysozyme, lactoferrin, -lactalbumin) which have important nutraceutical properties, and by a high content of vitamin C. However, the main problem on the use of DM is its poor availability (responsible of the high cost per liter of this food) which is limited to few months per year, because the ass’s fertility is strictly connected with photoperiod. At this purpose, it is important to find valid long-term storage conditions that keep unaltered the nutraceutical properties of this milk. In this work, the whey protein fraction (normally used to study the impact of a thermal treatment on milk) and the vitamin C content has been analyzed after some thermal treatments on donkey milk such as freezing (-20°C), lyophilization (freeze-drying), and spray-drying technique. Three representative whey proteins, β-lactoglobulin, α-lactalbumin, and lysozyme, from fresh DM, frozen milk (-20°C) and both reconstituted freeze-dried and spray-dried DM have been analyzed by RP-HPLC, dynamic light scattering analyses and differential scanning microcalorimetry experiments. The enzymatic activity of lysozyme has been also performed since it can represent a valuable index of the nutritional quality of the processed milk. Results obtained were compared with those determined on fresh milk. RP-HPLC analyses revealed that the freeze-drying and spray-drying processes reduced significantly (P < 0.05) β-lactoglobulin concentration whereas lysozyme maintains 96% and 58% residual activity in the freeze-dried milk and spray-dried DM, respectively [4]. The content of α-lactalbumin was not affected by these two drying processes. Differential scanning microcalorimetry and dynamic light scattering analyses were performed to investigate the thermal behavior of lysozyme, β-lactoglobulin and α-lactalbumin from fresh, freeze-dried and spray-dried DM, revealing no effect on the thermal behavior of whey proteins. When DM is maintained at -20° for up to three months α-lactalbumin and β-lactoglobulin content resulted quite stable. The lysozyme activity, in fresh as well as in frozen milk, resulted to be 0.035 U/ml, indicating that the freezing and thawing out process do not affect the activity of this important enzyme. L-Ascorbic acid (vitamin C) can be used as a quality indicator in the production of several foods and derivatives such as wine, beer, milk, soft drinks and fruit juices. Fresh DM contains 57 mg of vitamin C per liter of milk. The freeze-drying process did not significantly affect the amount of vitamin C in DM but its content significantly decreased (P<0.01) after one, two and three months of storage at -20°C. In conclusion, the spray-drying process caused a significant decrease of DM lysozyme enzymatic activity and of β-lactoglobulin total amount, because of the high temperature to which milk is subjected during this treatment. Freeze-drying treatment demonstrated that the nutritional characteristics of DM remained basically unchanged if compared with fresh DM. References [1] Carroccio A, Cavataio F, Montalto G, D'Amico D, Alabrese L. Intolerance to hydrolysated cow's milk proteins in infants: Characteristics and dietary treatment. Clin Exp Allergy. 2000;30:1597-1603. [2] Salimei E, Fantuz F, Coppola R, Chiofalo B, Polidori P, Varisco G. Composition and characteristics of ass's milk. Animal Research. 2004;53:67-68. [3] Vincenzetti S, Polidori P, Mariani P, Cammertoni N, Fantuz F, Vita A. Donkey's milk protein fraction characterization. Food Chemistry. 2008;106:640-649. [4] Vincenzetti S, Cecchi T, Perinelli DR, Pucciarelli S, Polzonetti V, Bonacucina G, Ariani A, Parrocchia L, Spera DM, Ferretti E, Vallesi P, Polidori P. Effects of freeze-drying and spray- drying on donkey milk volatile compounds and whey proteins stability. LWT - Food Sci Technol. 2018; 88: 189-195.